Problems with Heaters
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any clues?
At this stage, I think we can rule-out interference as a reason.
Obviously, the two thermistor signals of bed and hotend are related in a way that a drop in the readings from the hotend results in a rise of the bed’s signal. Further to note: this "relationship" is intermittent, I.e. the readings are not coupled all of the time.
The latter observation tells us that we face an irregularity: either a bad crimp, broken wire, faulty connector, or a temporary short with GND, VIN (any voltage), or another signal.
As both readings are affected when this irregularity happens, a single intermittent signal line is unlikely to cause this effect. This leaves us with just two possible culprits:
Either an intermittent short - along the lines, on/below the board or to metal printer parts (that’s @DC’s most prominent guess) …
Or some unstable voltage on a region of the board - either there’s something with the 3.3V rail, insufficient 5V power or improper GND. That's just a faint suspicion - I'm no expert at all.
To narrow down on the culprit, you can disconnect single components from the Duet, one at a time (and only with an unpowered board): replace the thermistors (one at a time) with a 100k resistor - for that, you can solder a plug with the resistor across the two pins.
Then, you can disconnect the bed heater from the board, both lines. However, if you bed is mains powered, you must remove those lines, too. Only try this if you know what you do, else, your life is in danger. Seriously.
Separate the hotend with heater and thermistor from the print head, but keep the lines connected. Put it on some ceramic plate so that you can run it without fire hazard. If that works, you know that you have a GND problem with metal printer parts.
Another way to prove this is to carefully ground all parts of you printer, including the hotend. I think you should do this anyway.
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Thanks for the detailed analysis. I tried the following, and need to do more but I've run out of time for today and wont be able to get back to it for another 24 hours, so I wanted to get this out there:
- Plugged old thermistor (in lieu of 100k resistor) into bed thermistor socket. Problem still occurred when attempt made to heat the hotend.
- Hotend removed and ground wire connected to heat block. Problem still occurred when attempt made to heat the hotend.
For the next step, of component disconnection, I'm not sure what will happen when I try removing components from the board, but wouldn't it generally show up as fault when reconnected? I'm not good on electronics or software, so your patience is appreciated!
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@tomf First, I would like to know what conditions you need in order to see the "problem": Obviously, you have to fire up the hotend. But do you need to heat the bed, too? Does the error only show up during a print? Or must the print head be in a specific location, must it be moving?
Second, please explain how you note the error: can you just see the irregularities in DWC's temperature plot, or are there other symptoms? Maybe a dimmed LED on the board, an error or warning in the console? …
I just want to learn more about the path towards the error. How many variables are required? The less, the better. In a second step, it would be nice to establish a simple and repeatable procedure for testing.
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@tomf I suggest you replace both the hot end thermistor and the hot end heater cartridge. I think one or both of them have an intermittent short between the element and case when hot. I assume you have already checked the hot end thermistor wires for possible shorting.
Duet WiFi hardware designer and firmware engineer
Please do not ask me for Duet support via PM or email, use the forum
http://www.escher3d.com, https://miscsolutions.wordpress.com -
@infiniteloop said in Problems with Heaters:
@tomf First, I would like to know what conditions you need in order to see the "problem": Obviously, you have to fire up the hotend. But do you need to heat the bed, too? Does the error only show up during a print? Or must the print head be in a specific location, must it be moving?
Second, please explain how you note the error: can you just see the irregularities in DWC's temperature plot, or are there other symptoms? Maybe a dimmed LED on the board, an error or warning in the console? …
I just want to learn more about the path towards the error. How many variables are required? The less, the better. In a second step, it would be nice to establish a simple and repeatable procedure for testing.
I set the hotend first (typically 235C) and observe the chart.
If the printer's been switched off for a long period (more than 2 hours), the temperature rises normally and is steady. I then switch on the bed (60C) and that is also steady. When I try to do anything (extrude material, home axes typically) the problem then happens, as seen in the temperature plots.
After the first incident of the day, the problem recurrs without the bed heater being switched on and without needing to do anything else. It doesn't matter where the print head is. The bed can be cold to the touch throughout.
When the problem occurs there is a fault message and heaters are automatically disabled.
There are no other symptoms, at least none that I have seen.
The problem has been getting worse, evidenced by taking less time to occur.
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@tomf Thx for the details. Looks like the initial procedure involves more steps than subsequent trials:
If the printer's been switched off for a long period (more than 2 hours), the temperature rises normally and is steady. I then switch on the bed (60C) and that is also steady. When I try to do anything (extrude material, home axes typically) the problem then happens, as seen in the temperature plots.
Please perform two tests with this scenario:
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Can you omit the step of heating the bed? Does it make a difference?
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Instead of homing the axes (which, if I understand you right, provokes the error), try to move the wiring harness for the print head into different directions: does that trigger the error, too?
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@dc42 said in Problems with Heaters:
@tomf I suggest you replace both the hot end thermistor and the hot end heater cartridge. I think one or both of them have an intermittent short between the element and case when hot. I assume you have already checked the hot end thermistor wires for possible shorting.
I agree that the problem could be the thermistor or heater (or maybe the Duet). Unfortunately I don't have these as spares and although I've ordered replacements, they will take about 3 weeks to get here. So I would like to confirm that somehow in case there's something else going on. The hotend heater measures 12.5 ohms, and the bed 25 ohms.
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@infiniteloop Will do when I get back later.
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@tomf said in Problems with Heaters:
The hotend heater measures 12.5 ohms, and the bed 25 ohms.
So I can assume your PSU has 24V, and your bed is not powered from mains?
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@infiniteloop said in Problems with Heaters:
@tomf said in Problems with Heaters:
The hotend heater measures 12.5 ohms, and the bed 25 ohms.
So I can assume your PSU has 24V, and your bed is not powered from mains?
Yes, 24V supply, but the bed is mains powered,120V, 600W, through SSR.
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@tomf said in Problems with Heaters:
Yes, 24V supply, but the bed is mains powered,120V, 600W, through SSR.
Good to know.
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Following up:
The spare parts arrived yesterday and today I replaced just the hotend thermistor. That seems to have resolved the problem as I did about 6 hours of printing with no events. Yipee!
I remain unclear how a faulty hotend thermistor could have had consequential effects on the reported bed temperature as shown in the chart (from the original problem) so any celebrations might be premature, but no such problem today.
